The purpose of this study is to test the hypothesis that human milk beta-glucuronidase activity varies among races and is one factor related to the interracial differences in neonatal hyperbilirubinemia. A clinical protocol involving White, Asian, and Black newborns (fifteen in each group) who are exclusively fed human milk, will compare beta-glucuronidase levels in milk and feces during the first week of life. This information will be related to levels of serum bilirubin.